Use of dialkylene glycol ester to increase the oxidation resistance of a lubricant composition

ABSTRACT

The present invention relates to the use of at least one diester having formula (I) in a lubricant composition containing at least one base oil, so as to increase the oxidation resistance of said lubricant composition or to decrease the oxidation level of said lubricant composition, 
       R a —C(O)—O—([C(R) 2 ] n —O) s —C(O)—R b    (I)

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a U.S. National Phase Application under 35 U.S.C. § 371 of International Patent Application No. PCT/EP2021/068857 filed Jul. 7, 2021, which claims priority of French Patent Application No. 20 07286 filed Jul. 9, 2020. The entire contents of which are hereby incorporated by reference.

FIELD OF THE INVENTION

The present invention relates to the use of dialkylene glycol ester to increase the oxidation resistance of a lubricant composition.

BACKGROUND

Lubricant compositions also called «lubricants» are commonly used in engines for the chief purpose of reducing friction forces between the different moving metal parts inside engines. They are also efficient in preventing early wear, even damage to parts, in particular the surface thereof.

One of the main objectives in the automotive field is to endeavour to increase the service life of lubricant compositions and thereby to obtain longer intervals between oil change times.

Lubricant compositions are subjected to numerous stresses, particularly mechanical stresses, but also oxidation stresses. These oxidation stresses are mainly due to contacts between the lubricant compositions and air and//or water, and to changes in temperature of the lubricant composition.

There would therefore be an advantage in improving the oxidation resistance of lubricant compositions.

SUMMARY

It is therefore one objective of the present invention to propose an additive allowing an increase in the oxidation resistance of a lubricant composition.

A further objective of the present invention is also to propose an additive allowing a decrease in the oxidation level of a lubricant composition.

A further objective is to provide an additive allowing the service life of a lubricant composition to be increased, and hence to obtain longer intervals between oil change times.

Other objectives will become apparent on reading the following description of the invention.

These objectives are reached with the present invention which relates to the use of at least one diester of formula (I) in a lubricant composition comprising at least one base oil, to increase the oxidation resistance of said lubricant composition,

R^(a)—C(O)—O—([C(R)₂]_(n)—O)_(s)—C(O)—R^(b)   (I)

-   -   where:     -   R, each independently, are a hydrogen atom or a linear or         branched alkyl group having 1 to 5 carbon atoms, in particular a         methyl, ethyl or propyl group, preferably methyl;     -   s is 1 or 2;     -   n is 1, 2 or 3, on the understanding that when s differs from 1,         the n values can be the same or different;     -   Ra and Rb, the same or different, are each independently linear         or branched, saturated or unsaturated hydrocarbon groups having         a linear sequence of 6 to 18 carbon atoms;     -   provided that when s is 2 and n all the same are 2, at least one         of the R groups is a linear or branched alkyl group having 1 to         5 carbon atoms; and     -   provided that when s is 1 and n is 3, at least one of the R         groups linked to the carbon at beta position of the oxygen atoms         of the ester functions, represents a hydrogen atom.

The present invention also relates to the use of at least one diester of formula (I) in a lubricant composition comprising at least one base oil, to decrease the oxidation level of said lubricant composition,

R^(a)—C(O)—O—([C(R)₂]_(n)—O)_(s)—C(O)—R^(b)   (I)

-   -   where:     -   R, each independently, are a hydrogen atom or a linear or         branched alkyl group having 1 to 5 carbon atoms, in particular a         methyl, ethyl or propyl group, preferably methyl;     -   s is 1 or 2;     -   n is 1, 2 or 3, on the understanding that when s differs from 1,         the n values can be same or different;     -   Ra and Rb, the same or different, are each independently linear         or branched, saturated or unsaturated hydrocarbon groups having         a linear sequence of 6 to 18 carbon atoms;     -   provided that when s is 2 and n all the same are 2, at least one         of the R groups is a linear or branched alkyl group having 1 to         5 carbon atoms; and     -   provided that when s is 1 and n is 3, at least one of the R         groups linked to the carbon at beta position of the oxygen atoms         of the ester functions, represents a hydrogen group.

The present invention also relates to a method for increasing the oxidation resistance of a lubricant composition comprising at least one base oil, comprising the addition to said lubricant composition of at least one diester of formula (I)

R^(a)—C(O)—O—([C(R)₂]_(n)—O)_(s)—C(O)—R^(b)   (I)

-   -   where:     -   R, each independently, are a hydrogen atom or a linear or         branched alkyl group having 1 to 5 carbon atoms, in particular a         methyl, ethyl or propyl group, preferably methyl;     -   s is 1 or 2;     -   n is 1, 2 or 3, on the understanding that when s differs from 1,         the n values can be the same or different;     -   Ra and Rb, the same or different, are each independently linear         or branched, saturated or unsaturated hydrocarbon groups having         a linear sequence of 6 to 18 carbon atoms;     -   provided that when s is 2 and n all the same are 2, at least one         of the R groups is a linear or branched alkyl group having 1 to         5 carbon atoms; and     -   provided that when s is 1 and n is 3, at least one of the R         groups linked to the carbon at beta position of the oxygen atoms         of the ester functions, represents a hydrogen atom.

The present invention also relates to a method for decreasing the oxidation level of a lubricant composition comprising at least one base oil, comprising the addition to said lubricant composition of at least one diester of formula (I)

R^(a)—C(O)—O—([C(R)₂]_(n)—O)_(s)—C(O)—R^(b)   (I)

-   -   where:     -   R, each independently, are a hydrogen atom or a linear or         branched alkyl group having 1 to 5 carbon atoms, in particular a         methyl, ethyl or propyl group, preferably methyl;     -   s is 1 or 2;     -   n is 1, 2 or 3, on the understanding that when s differs from 1,         the n values can be the same or different;     -   Ra and Rb, the same or different, are each independently linear         or branched, saturated or unsaturated hydrocarbon groups having         a linear sequence of 6 to 18 carbon atoms;     -   provided that when s is 2 and n all the same are 2, at least one         of the R groups is a linear or branched alkyl group having 1 to         5 carbon atoms; and     -   provided that when s is 1 and n is 3, at least one of the R         groups linked to the carbon at beta position of the oxygen atoms         of the ester functions, represents a hydrogen atom.

In the present invention, it is to be understood that the increase in oxidation resistance and decrease (or lowering) of the oxidation level refer to a lubricant composition not comprising the diester of formula (I).

Modification of the oxidation level of a lubricant composition is determined by measuring the viscosity thereof and the change in this viscosity, for example KV100 or KV40. As and when the polar compounds of oxidation start to form, subsequent to entry of oxygenated compounds into the structure of the lubricant composition, the viscosity and/or acidity of the lubricant composition increase. It is therefore possible to determine the oxidation level of a lubricant composition comprising or not comprising the diester of the invention, and to determine the impact of this diester on the oxidation level and hence on oxidation resistance.

KV100 and KV40 can be measured in any manner known to skilled persons in accordance with standard ASTM445.

Preferably, in the diester of formula (I) of the invention, when s differs from 1, all the n values are the same.

Preferably, in the diester of formula (I) of the invention, n is 2 or 3, preferably 2.

Preferably, in the diester of formula (I) of the invention, at least one of the R groups is a linear or branched alkyl group having 1 to 5 carbon atoms, preferably 1 to 4 carbon atoms, more preferably methyl, ethyl or propyl, advantageously methyl.

In one particular embodiment, the diester of formula (I) of the invention is a diester of following formula (I′):

R^(a)—C(O)—O—([C(R)₂]_(n)—O)—([C(R′)₂]_(m)—O)_(s-1)—C(O)—R^(b)   (I′)

-   -   where:     -   R and R′ are each independently a hydrogen atom or a linear or         branched alkyl group having 1 to 5 carbon atoms, preferably a         methyl, ethyl or propyl group, preferably a methyl group;     -   s is 1 or 2;     -   n is 2;     -   m is 2;     -   Ra and Rb, the same or different, are each independently linear         or branched, saturated or unsaturated hydrocarbon groups having         a linear sequence of 6 to 18 carbon atoms;     -   provided that when s is 2, at least one of the groups R or R′ is         a linear or branched alkyl group having 1 to 5 carbon atoms.

Advantageously, at least one of the groups R or R′ in the diester of formula (I′) is a linear or branched alkyl group having 1 to 5 carbon atoms, preferably 1 to 4 carbon atoms, preferably methyl, ethyl or propyl, advantageously methyl.

Preferably, in the compounds of formula (I) and (I′), Ra and Rb have a linear sequence of 7 to 14 carbon atoms, preferably 8 to 12 carbon atoms, more particularly 8 to 11 carbon atoms, and in particular 9 or 11 carbon atoms.

In one particular embodiment, s is 2 in formulas (I) or (I′).

The diester of the invention is preferably a compound of formula (I′a):

R^(a)—C(O)—O—([C(R)₂]_(n)—O)—([C(R′)₂]_(m)—O)—C(O)—R^(b)   (I′a)

-   -   where:         -   R and R′ are each independently a hydrogen atom or a linear             or branched alkyl group having 1 to 5 carbon atoms,             preferably methyl, ethyl or propyl, preferably methyl;         -   n is 2;         -   m is 2;         -   Ra and Rb, the same or different, are each independently             linear or branched, saturated or unsaturated hydrocarbon             groups having a linear sequence of 6 to 18 carbon atoms;     -   provided that at least one of the groups R or R′ is a linear or         branched alkyl group having 1 to 5 carbon atoms, preferably         methyl, ethyl or propyl, preferably methyl.

Preferably, at least one of the R groups is a linear or branched alkyl group having 1 to 5 carbon atoms, preferably methyl, ethyl or propyl, preferably methyl; and at least one of R′ is a linear or branched alkyl group having 1 to 5 carbon atoms, preferably methyl, ethyl or propyl, preferably methyl.

Further preferably, in the diester of formula (l′a) one of the R groups is a linear or branched alkyl group having 1 to 5 carbon atoms, preferably methyl, ethyl or propyl, preferably methyl; and one of R′ is a linear or branched alkyl group having 1 to 5 carbon atoms, preferably methyl, ethyl or propyl, preferably methyl; the other R and R′ groups representing hydrogen atoms.

In one particular embodiment, the diester of the invention is a compound of formula (I″a)

R^(a)—C(O)—O—CHR¹—CHR²—O—CHR³—CHR⁴—O—C(O)—R^(b)   (I″a)

-   -   where:         -   one of the groups R¹ and R² is a linear or branched alkyl             group having 1 to 5 carbon atoms, the other representing a             hydrogen atom;         -   one of the groups R³ and R⁴ is a linear or branched alkyl             group having 1 to 5 carbon atoms, the other representing a             hydrogen atom;         -   Ra and Rb, the same or different, are such as previously             defined.     -   Preferably, in the compounds of formula (Fa):         -   one of the groups R¹ and R² is a methyl, ethyl or propyl             group, preferably methyl, the other representing a hydrogen             atom;         -   one of the groups R³ and R⁴ is a methyl, ethyl or propyl             group, preferably methyl, the other representing a hydrogen             atom.

In another alternative embodiment, in the compounds of formula (I) or (I′), s is 1 and the diesters of the invention have the formula (I′b):

R^(a)—C(O)—O—([C(R)₂]_(a)—O)—C(O)—R^(b)   (I′b)

-   -   where:         -   R, each independently, are a hydrogen atom or a linear or             branched alkyl group having 1 to 5 carbon atoms, preferably             methyl, ethyl or propyl, preferably methyl;         -   n is 2;         -   Ra and Rb, the same or different, are each independently             linear or branched, saturated or unsaturated hydrocarbon             groups having a linear sequence of 6 to 18 carbon atoms.

Preferably, in formula (I′b), at least one of R is a linear or branched alkyl group having 1 to 5 carbon atoms, preferably methyl, ethyl or propyl, preferably methyl.

Preferably, in formula (I′b), one of the R groups is a linear or branched alkyl group having 1 to 5 carbon atoms, preferably methyl, ethyl or propyl, preferably methyl, the others representing hydrogen atoms.

In the present invention, it is to be understood that the expression «having x to y carbon atoms» also includes the limits x and y.

In the present invention, by «linear sequence of x to y carbon atoms» it is to be understood a saturated or unsaturated carbon chain, preferably saturated, having x to y carbon atoms each following after each other, the carbon atoms possibly present at the branches of the carbon chain not being taken into account in the number of carbon atoms (x-y) forming the linear sequence.

In one particular embodiment, in formulas (I), (I′), (I′a), (I″a) or (I′b), Ra and Rb, the same or different, are of vegetable, animal or petroleum origin.

In one particular embodiment, in the formulas (I), (I′), (I′a), (I″a) or (I′b), Ra and Rb, the same or different, represent saturated groups.

In one particular embodiment, in formulas (I), (I′), (I′a), (I″a) or (I′b), Ra and Rb, the same or different, represent linear groups. In particular, Ra and Rb, the same or different, are saturated linear hydrocarbon groups having 6 to 18 carbon atoms, preferably 7 to 17 carbon atoms, in particular 7 to 14 carbon atoms, preferably 8 to 12 carbons atoms and particularly 9 or 12 carbon atoms.

In another preferred embodiment, in formulas (I), (I′), (I′a), (I″a) or (I′b), Ra and Rb, the same or different, are saturated linear alkyl groups having 6 to 18 carbon atoms, preferably 7 to 17 carbon atoms, in particular 7 to 14 carbon atoms, preferably 8 to 12 carbon atoms, and in particular 9 or 12 carbon atoms.

Preferably, Ra and Rb are the same

The diesters of formula (I) can be commercially available or prepared following the synthesis methods described in the literature and known to persons skilled in the art, in particular following the methods described in WO201925446.

Preferably, the diester of formula (I) is added in a proportion of 10 to 70% by weight relative to the total weight of the lubricant composition.

The base oil used in the lubricant compositions of the invention can be oils of mineral or synthetic origin belonging to Groups I to V of the classes defined by the API classification (or the equivalents thereof in the ATIEL classification (Table 1), or mixtures thereof.

TABLE 1 Content of Viscosity saturated Sulfur index substances content (VI) Group I <90% >0.03% 80 ≤ VI < 120 Mineral oils Group II ≥90% ≤0.03% 80 ≤ VI < 120 Hydrocracked oils Group III ≥90% ≤0.03% ≥120 Hydro-isomerized oils Group IV Polyalphaolefins (PAOs) Group V Esters and other bases not included in Groups I to IV

The mineral base oils of the invention include any type of base oil obtained by atmospheric and vacuum distillation of crude oil, followed by refining operations such as solvent extraction, deasphalting, solvent dewaxing, hydrotreatment, hydrocracking, hydroisomerization and hydrofinishing.

Mixtures of synthetic and mineral oils can also be used.

The base oils of the lubricant compositions of the invention can also be selected from among synthetic oils such as some esters of carboxylic acids and of alcohols, and polyalphaolefins. The polyalphaolefins used as base oil are obtained for example from monomers having 4 to 32 carbon atoms, for example from octene or decene, and in which viscosity at 100° C. is between 1.5 and 15 mm²·s⁻¹ according to standard ASTM D445. They generally have a molecular weight average of between 250 and 3000 according to standard ASTM D5296.

The lubricant composition of the invention may comprise at least 50% by weight of base oil relative to the total weight of the composition. More advantageously, the lubricant composition of the invention comprises at least 60% or even at least 70% by weight of base oils relative to the total weight of the lubricant composition. More preferably, the lubricant composition of the invention comprises 75 to 95% by weight of base oils relative to the total weight of the composition.

The composition of the invention may also comprise at least one additive.

Numerous additives can be used in the lubricant compositions of the invention.

The preferred additives for the lubricant composition of the invention are selected from among detergent additives, friction modifier additives differing from the molybdenum compounds defined above, extreme pressure additives, dispersants, pour point activators, defoaming agents, thickeners and mixtures thereof.

Preferably, the lubricant compositions of the invention comprise at least one extreme pressure additive, or a mixture.

The anti-wear additives and extreme pressure additives protect against surface frictions by forming a protective film adsorbed on the surfaces thereof.

There exists a wide variety of anti-wear additives. Preferably, for the lubricant compositions of the invention, the anti-wear additives are selected from among additives comprising phosphorus and sulfur such as alkylthiophosphate metals, in particular zinc alkylthiophosphate, and more specifically zinc dialkyldithiophosphate or ZnDTP. The preferred compounds have the formula Zn((SP(S)(OR)(OR′))2, where R and R′, the same or different, are each independently an alkyl group, preferably an alkyl group having 1 to 18 carbon atoms.

Amine phosphates are also anti-wear additives which can be used in the lubricant compositions of the invention. However, the phosphorus atoms contributed by these additives may act as poison for the catalytic systems of automotive vehicles since they generate ash. It is possible to minimise these effects by substituting part of the amine phosphates by additives not contributing phosphorus such as polysulfides, in particular sulfur-containing olefins.

Advantageously, the lubricant compositions of the invention may comprise from 0.01 to 6% by weight, preferably 0.05 to 4% by weight, more preferably 0.1 to 2% by weight, relative to the total weight of the lubricant composition, of anti-wear and extreme pressure additives.

Advantageously, the lubricant compositions of the invention comprise from 0.01 to 6 by weight, preferably 0.05 to 4% by weight, more preferably 0.1 to 2% by weight, relative to the total weight of the lubricant composition of anti-wear additives (or anti-wear compound).

Advantageously, the compositions of the invention may comprise at least one friction modifier additive differing from the molybdenum compounds of the invention. The friction-modifier additives can be selected in particular from among compounds contributing metallic elements and ash-free compounds. Among compounds contributing metal elements, mention can be made of complexes of transition metals such as Mo, Sb, Sn, Fe, Cu, Zn in which the ligands can be hydrocarbon compounds comprising oxygen, nitrogen, sulfur or phosphorus atoms. Ash-free friction modifier additives are generally of organic origin or can be selected from among fatty acid and polyol monoesters, alkoxylated amines, alkoxylated fatty amines, fatty epoxides, fatty epoxide borates, fatty amines or glycerol acid esters. In the invention, the fatty compounds comprising at least one hydrocarbon group have 10 to 24 carbon atoms.

Advantageously, the lubricant composition of the invention may comprise from 0.01 to 2% by weight or 0.01 to 5% by weight, preferably 0.1 to 1.5% by weight or 0.1 to 2% by weight, relative to the total weight of the lubricant composition, of friction modifier additive differing from the molybdenum compounds of the invention.

Advantageously, the lubricant composition of the invention may comprise at least one antioxidant additive.

Antioxidant additives generally delay degradation of the lubricant composition. Most often this degradation translates as the formation of a deposit, the presence of sludge or by an increase in viscosity of the lubricant composition.

Antioxidant additives generally act as radical inhibitors or hydroperoxide decomposers. Among the antioxidants commonly used, mention can be made of antioxidants of phenolic type, antioxidants of amine type, antioxidants containing sulfur and phosphorus. Some of these antioxidants, for example those comprising sulfur and phosphorus, can generate ash. Phenolic antioxidant additives can be ash-free or can be in the forms of neutral or basic metal salts. The antioxidant additives can be selected in particular from among sterically hindered phenols, sterically hindered phenol esters, sterically hindered phenols comprising a thioether bridge, diphenylamines, diphenylamines substituted with at least one C1 to C12 alkyl group, N,N′-dialkyl-aryl-diamines, and mixtures thereof.

Preferably, in the invention, the sterically hindered phenols are selected from among compounds comprising a phenol group in which at least one of the carbon atoms in the vicinity of the carbon atom carrying the alcohol function is substituted by at least one C1 to 010 alkyl group, preferably a C1 to C6 alkyl group, more preferably a C4 alkyl group, preferably a tert-butyl group.

Amine compounds form another class of antioxidant additives which can be used, optionally in combination with phenolic antioxidant additives. Examples of amine compounds are aromatic amines e.g. aromatic amines having the formula NRaRbRc in which Ra is an optionally substituted aliphatic group or aromatic group, Rb is an optionally substituted aromatic group, Rc is a hydrogen atom, an alkyl group, an aryl group or a group of formula RdS(O)zRe where Rd is an alkylene or alkenylene group, Re is an alkyl group, an alkenyl group or an aryl group and z is 0, 1 or 2.

Alkyl-phenols containing sulfur or the alkali or alkaline-earth metal salts thereof can also be used as antioxidant additives.

Other classes of antioxidant additives are copper-containing compounds, for example copper thio- or dithio-phosphates, copper salts and carboxylic acids, dithiocarbamates, sulfonates, phenates, copper acetylacetonates. The salts of copper I and II, the salts of succinic acid or anhydride can also be used.

The lubricant compositions of the invention may also comprise any type of antioxidant known to skilled persons.

Advantageously, the lubricant composition comprises at least one ash-free antioxidant additive.

Also advantageously, the lubricant composition of the invention comprises from 0.1 to 2% by weight of at least one antioxidant additive, relative to the total weight of the composition.

The lubricant composition of the invention may also comprise at least one detergent additive.

Detergent additives generally allow reduced forming of deposits on the surface of the metal parts, by dissolving secondary products of oxidation and combustion.

The detergent additives able to be used in the lubricant compositions of the invention are generally known to those skilled in the art. The detergent additives can be anionic compounds having a long lipophilic hydrocarbon chain and a hydrophobic head. The associated cation can be a metal cation of an alkali or alkaline-earth metal.

The detergent additives are preferably selected from among alkali or alkaline-earth metal salts of carboxylic acid, sulfonates, salicylates, naphthenates, and phenate salts. The alkali and alkaline-earth metals are preferably calcium, magnesium, sodium or barium.

These metal salts generally comprise the metal in stoichiometric amount or in excess i.e. in an amount greater than the stoichiometric content. These are then overbased detergents: the excess metal imparting the overbased nature to the detergent additive is generally in the form of an oil-insoluble metal salt e.g. a carbonate, hydroxide, oxalate, acetate, glutamate, preferably carbonate.

Advantageously, the lubricant composition of the invention may comprise from 0.5 to 8% or 2 to 4% by weight of overbased detergent additive, relative to the total weight of the lubricant composition.

Also advantageously, the lubricant composition of the invention may also comprise a pour point depressant additive.

By slowing the formation of paraffin crystals, a pour point depressant additive generally improves the cold start behaviour of the lubricant composition of the invention.

As examples of pour point depressant additives, mention can be made of polymethacrylate alkyls, polyacrylates, polyarylamides, polyalkylphenols, polyalkylnaphthalene, polystyrene alkyls.

Advantageously, the lubricant composition of the invention may also comprise a dispersant.

The dispersants can be selected from among Mannich bases, succinimides and derivatives thereof.

Also advantageously, the lubricant composition of the invention may comprise from 0.2 to 10% by weight of dispersant, relative to the total weight of lubricant composition.

Advantageously, the lubricant composition of the invention may also comprise at least one other additional polymer improving the viscosity index. As examples of additional polymer improving the viscosity index, mention can be made of polymeric esters, homopolymers or copolymers, whether or not hydrogenated, of styrene, butadiene and isoprene, polymethacrylates (PMAs). Also advantageously, the lubricant composition of the invention may comprise from 1 to 15% by weight, relative to the total weight of the lubricant composition, of additive improving the viscosity index.

The lubricant composition of the invention may also comprise at least one thickening agent.

The lubricant composition of the invention may also comprise a defoaming agent and demulsifying agent.

Preferably, the lubricant composition of the invention further comprises at least one anti-wear agent, in particular zinc-based and particularly ZnDTP.

The present invention also concerns the use of the lubricant composition of the invention to reduce friction of the mechanical parts of an engine, at least one of the parts comprising a coating of amorphous carbon type, preferably hydrogenated amorphous carbon.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 gives the KV40 values as a function of time for the compositions of the invention and the comparative compositions.

FIG. 2 gives the KV100 values as a function of time for the compositions of the invention and the comparative compositions.

DETAILED DESCRIPTION

The present application is now described with the aid of nonlimiting examples.

Example 1: Lubricant Compositions

The lubricant compositions described in Table 2 were prepared (CC2, CC3, CC4 and CC5 are comparative compositions, C2 and C3 are compositions of the invention).

TABLE 2 C2 CC2 CC3 C3 CC4 CC5 wt. (%) (wt. %) (wt. %) (wt. %) (wt. %) (wt. %) Additive 12.7  12.7  12.7  12.7  12.7  12.7  package (comprising detergent(s), dispersant(s) & antioxidant(s)) Antioxidant 0.5 0.5 0.5 0.5 0.5 0.5 MoDTC 0.5 0.5 0.5 0.5 0.5 0.5 Viscosity 4.4 4.4 4.4 4.4 4.4 4.4 modifiers Base oil KV100: 20   18   81.7  31.7  30   42.7  4.181 mm2/s Base oil KV100: 43.7  2.978 mm2/s Base oil KV100: 35   51.7  39   3.317 mm2/s Base oil KV100: 10   4.026 mm2/s Base oil KV100: 10   5.919 mm2/s C9/C12 diester 61.7  15   of propylene glycol V1-258 0.2 0.2 0.2 0.2 0.2 0.2

Example 2: Characteristics of the Lubricant Compositions

Table 3 below groups together the characteristics of the lubricant compositions in Example 1.

The KV100 and KV40 values were measured according to standard ASTMD445 and expressed in mm2/s.

VI was measured according to standard ISO2909.

HTHS was measured according to standard CEC L-036 and expressed in mPa·s

TABLE 3 C2 CC2 CC3 C3 CC4 CC5 BOV (Base oil viscosity calculated 3.6 3.6 4.2 3.6 3.6 3.8 taking into account the mixture of viscosities and proportions of base oils contained in the composition) Noack (CEC L-40-A-93) 11.7 24.5 11.7 18.4 21.1 18.6 KV100 6.66 7.405 8.272 7.285 7.439 7.638 KV40 27.9 36.82 42.18 34.57 36.75 38.03 VI 209 172 176 183 174 175 HTHS at 150° Ravenfield 2.44 2.36

Example 3: Impact of the Diester of the Invention on Oxidation

The KV100 and KV40 values of compositions C2, CC2, CC3, C3, CC4 and CC5 were measured as a function of time according to standard ASTMD445. Measurements were taken after 3 days, 4 days, 5 days, 6 days, 7 days, 8 days and 9 dawns with the following protocol:

In a container containing the compositions to be tested, air was injected at a flow rate of 10 L·h−1 throughout the entire duration of the test. Iron was also added in an amount of 100 ppm at T0 to stimulate/catalyse oxidation of the tested compositions. In addition, the container containing the tested compositions was left throughout the entire duration of the test over a hot water bath heated to 170° C.

The results are given in FIGS. 1 and 2 .

As and when the polar compounds of oxidation started to form, after entry of the oxygenated compounds into the structure of the lubricant composition, the viscosity of the lubricant composition increased.

FIGS. 1 and 2 clearly show that the addition of the diester of the invention allows limiting and delaying of the increase in viscosity of the lubricant composition. Therefore, the diester of the invention allows an increase in oxidation resistance and decrease (or lowering) of the oxidation level thereof. 

1.-8. (canceled)
 9. A method to increase the oxidation resistance of a lubricant composition comprising at least one base oil, comprising the addition to said lubricant composition of at least one diester of formula (I) R^(a)—C(O)—O—([C(R)₂]_(n)—O)_(s)—C(O)—R^(b)   (I) where: R, each independently, are a hydrogen atom or a linear or branched alkyl group having 1 to 5 carbon atoms; s is 1 or 2; n is 1, 2 or 3, on the understanding that when s differs from 1, the n values can be the same or different; Ra and Rb, the same or different, are each independently linear or branched, saturated or unsaturated hydrocarbon groups having a linear sequence of 6 to 18 carbon atoms; provided that when s is 2, and n all the same are 2, at least one of the R groups is a linear or branched alkyl group having 1 to 5 carbons atoms; and provided that when s is 1, and n is 3, at least one of the R groups linked to the carbon at beta position of the oxygen atoms of the ester functions, represents a hydrogen atom.
 10. A method for decreasing the oxidation level of a lubricant composition comprising at least one base oil, comprising the addition to said lubricant composition of at least one diester of formula (I) R^(a)—C(O)—O—([C(R)₂]_(n)—O)_(s)—C(O)—R^(b)   (I) where: R, each independently, are a hydrogen atom or a linear or branched alkyl group having 1 to 5 carbon atoms; s is 1 or 2; n is 1, 2 or 3, on the understanding that when s differs from 1, the n values can be the same or different; Ra and Rb, the same or different, are each independently linear or branched, saturated or unsaturated hydrocarbon groups having a linear sequence of 6 to 18 carbon atoms; provided that when s is 2, and n all the same are 2, at least one of the R groups is a linear or branched alkyl group having 1 to 5 carbon atoms; and provided that when s is 1, and n is 3, at least one of the R groups linked to the carbon at beta position of the oxygen atoms of the ester functions, represents a hydrogen atom.
 11. The method of claim 9, wherein when s differs from 1 all the n values are the same.
 12. The method of claim 9, wherein n is 2 or
 3. 13. The method of claim 9, wherein at least one of the R groups is a linear or branched alkyl group having 1 to 5 carbon atoms.
 14. The method of claim 9, wherein the diester of formula (I) of the invention is a diester of following formula (I′): R^(a)—C(O)—O—([C(R)₂]_(n)—O)—([C(R′)₂]_(m)—O)_(s-1)—C(O)—R^(b)   (I′) where: R and R′ each independently are a hydrogen atom or a linear or branched alkyl group having 1 to 5 carbon atoms; s is 1 or 2; n is 2; m is 2; Ra and Rb, the same or different, are each independently linear or branched, saturated or unsaturated hydrocarbon groups having a linear sequence of 6 to 18 carbon atoms; provided that when s is 2, at least one of the groups R or R′ is a linear or branched alkyl group having 1 to 5 carbon atoms.
 15. The method of claim 9, wherein the diester is a compound of formula (I″a) R^(a)—C(O)—O—CHR¹—CHR²—O—CHR³—CHR⁴—O—C(O)—R^(b)   (I″a) where: one of the groups R¹ and R² is a linear or branched alkyl group having 1 to 5 carbon atoms, the other representing a hydrogen atom; one of the groups R³ and R⁴ is a linear or branched alkyl group having 1 to 5 carbon atoms, the other representing a hydrogen atom.
 16. The method of claim 9, wherein the diester of formula (I) is added in a proportion of 10 to 70% by weight relative to the total weight of the lubricant composition.
 17. The method of claim 10, wherein when s differs from 1 all the n values are the same.
 18. The method of claim 10, wherein n is 2 or
 3. 19. The method of claim 10, wherein at least one of the R groups is a linear or branched alkyl group having 1 to 5 carbon atoms.
 20. The method of claim 10, wherein the diester of formula (I) of the invention is a diester of following formula (I′): R^(a)—C(O)—O—([C(R)₂]_(n)—O)—([C(R′)₂]_(m)—O)_(s-1)—C(O)—R^(b)   (I′) where: R and R′ each independently are a hydrogen atom or a linear or branched alkyl group having 1 to 5 carbon atoms; s is 1 or 2; n is 2; m is 2; Ra and Rb, the same or different, are each independently linear or branched, saturated or unsaturated hydrocarbon groups having a linear sequence of 6 to 18 carbon atoms; provided that when s is 2, at least one of the groups R or R′ is a linear or branched alkyl group having 1 to 5 carbon atoms.
 21. The method of claim 10, wherein the diester is a compound of formula (I″a) R^(a)—C(O)—O—CHR¹—CHR²—O—CHR³—CHR⁴—O—C(O)—R^(b)   (I″a) where: one of the groups R¹ and R² is a linear or branched alkyl group having 1 to 5 carbon atoms, the other representing a hydrogen atom; one of the groups R³ and R⁴ is a linear or branched alkyl group having 1 to 5 carbon atoms, the other representing a hydrogen atom.
 22. The method of claim 10, wherein the diester of formula (I) is added in a proportion of 10 to 70% by weight relative to the total weight of the lubricant composition. 